<!DOCTYPE html>
<html>

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title> </title>
    <style>
        .box {
            height: 300px;
            width: 300px;
            background-color: #ddd;
            border: 5px solid red;
            /**/
            border-bottom-style: dashed;
            /*线的类型dashed虚线  double双实线*/
            /* border-width: 10px;都是
            border-width:20px 30px; 垂直 水平
            border-width:20px 30px 5px;上 水平 下
            
           border-width:10px 10px 30px 40px; 顺时针 
            border-top-width: 10px; 
            border-top-color: chartreuse;和宽度类似*/
            margin: 40px auto;
         
        }

        .child {
            width: 100px;
            height: 100px;
            border: 5px solid yellow;
            background-color: blue;
            color: white;
            margin: 40px;
            /**/
            padding: 25px;
        }
    </style>
</head>

<body>
    <div class="box">
        测试
       <div class="child">你好</div> 
    </div>
</body>

</html>